Quantitative evaluation of the determinants of resolution in time-of-flight spectrometers for medium energy ion beam analysis

The determinants of resolution of a time-of-flight spectrometer for medium energy ion beam analysis have been identified and analyzed. The primary det erminants are uncertainty of ion path length, kinematic dispersion from the finite detector solid angle, straggling in the start foil and variability of the start foil thickness. The first three of these have been considered in previous studies of time-of-flight spectrometer resolution, but foil non -uniformity has not been examined in detail. Using backscattering analysis and atomic force microscopy, we have measured the thickness and roughness o f carbon start foils and found them to be larger than suggested by their no minal specifications. As a result, energy uncertainty introduced by foil no n-uniformity has been found to be a critical factor in determining resoluti on. Using measured values of foil parameters and known geometric characteri stics of our spectrometer, Monte Carlo simulations of backscattering spectr a of SiO2 thin films on Si substrates have been computed and found to repro duce well the experimentally observed system resolution of 1350 eV for 104 keV He. Additional simulations show that spectrometer design changes could reduce this value to about 1 keV at which point it is, for all practical pu rposes, optimum. (C) 1999 Elsevier Science B.V. All rights reserved.